Add a GitHub action to lint the Markdown.

Add a Markdownlint config file.

https://www.npmjs.com/package/markdownlint-cli

.github/workflows/lint.yml

@@ -0,0 +1,15 @@
+name: Lint Markdown
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: '12.x'
+      - run: npm install -g [email protected]
+      - run: markdownlint '**/*.md' --ignore node_modules

.markdownlint.yml

@@ -0,0 +1,32 @@
+# MD001 Header levels should only increment by one level at a time
+MD001: false
+
+# MD004 Unordered list style
+MD004: false
+
+# MD005 Inconsistent indentation for list items at the same level
+MD005: false
+
+# MD006 Consider starting bulleted lists at the beginning of the line
+MD006: false
+
+# MD007 Unordered list indentation
+MD007: false
+
+# MD013 Line length
+MD013: false
+
+# MD024 Multiple headers with the same content
+MD024: false
+
+# MD025 Multiple top level headers in the same document
+MD025: false
+
+# MD032 Lists should be surrounded by blank lines
+MD032: false
+
+# MD033 Inline HTML
+MD033: false
+
+# MD040 Fenced code blocks should have a language specified
+MD040: false